Dense Wavelength Division Multiplexing (DWDM) is the accepted solution for increasing telecommunication network capacity, while controlling overall system cost. Currently, there are several manufacturers that offer multi-wavelength WDM systems using channel rates up to 10 GB/S. Some of these systems are scalable up to 400 GB/s at 0.4 nm channel spacing.
The wavelength allocations for the systems in deployment today all follow the ITU recommendation of 193.1 THz±100 GHz. Currently, manufacturers offer a variety of channel spacing such as 200 GHz (1.6 nm), 100 GHz (0.8 nm), and 50 GHz (0.4 nm).
The high bandwidth of the optical fiber, as well as the bandwidth of erbium-doped fiber amplifiers, enable DWDM. Amplifier bandwidth is usually the flat region of 1540 to 1560 nm, or about 1525 to 1565 nm for gain-flattened amplifiers. This demand for bandwidth has further stimulated research on extended-bandwidth amplifiers, which are reported to have bandwidths of about 1530 to 1610 nm.
Most of the installed fiber network lines assume a point to point link between sites. This architecture is being rapidly extended to include add/drop capability for channel wavelengths using switches and routers that control channel destination. Such desired network flexibility requires monitoring for operation and management. For example, a change of the power of an added channel may degrade Signal to Noise Ratio (SNR) of other channels, or alternately, a rerouted wavelength may not have the needed SNR to carry traffic if injected into routes that do not have ample safety margins.
Furthermore, monitoring the status of DWDM channels has become a requirement to minimize network down time, as well as to initiate preventive measures, such as aging or drift of individual transmitters. Alternatively, environmental conditions or damage to the fiber cable may degrade some or all transmissions. Such a variety of events require the network manager to monitor all network-operating conditions simultaneously. Knowing the location and source of a fault goes a long way to minimizing repair time or the number of affected calls.
The most significant parameters that are required in channel monitoring are channel power, SNR and wavelength. Channel power and SNR are affected by the accumulation of insertion loss, polarization dependent loss (PDL), amplifier gain, and other effects, of the various in-line components in the network. Channel wavelength is driven by the transmitter's wavelength. If the wavelength drifts beyond its specifications, which are very tight for 50 GHz channel spacing, it contributes to cross talk and its failure, as well as neighboring channels.
There are other parameters that may be measured out in the field, such as cross talk and amplifier gain; however, the above three are the most important in a DWDM system. Other network requirements for optical monitoring are long operating lifetime, minimal servicing, low cost and integration into the network management system via the supervisory channel.
FIG. 1 shows a schematic of a DWDM network 110 including optical network monitors 112 for monitoring the status of all channels on the optical network 122. A transmitter node 114 transmits data at various wavelengths 116, 118 onto the optical network 122. An intermediate node or mid-node 124 may be positioned on the optical network 122 for adding or dropping data from the network 122. The network 122 terminates in a receiver node 126. It is very efficient to monitor the network at multiple nodes throughout the network to ensure accurate transmission of data. FIG. 1 shows three ONMs 112 for monitoring the network at various locations to ensure accurate initial transmission, accurate adding or dropping of data and accurate receiving of data.
There are a variety of prior art approaches to spectral analysis, particularly as applied to DWM networks. Each has its own merits and shortcomings. These prior art approaches are summarized in FIGS. 2a–d. 
Rotating Grating/Fixed Detector OSA, Shown in FIG. 2a.                 This configuration has a rotating grating, which allows for wide spectral range (600 nm to 1700 nm). As shown in FIG. 2a, the approach also accommodates a double-pass over the grating, which gives the signal. dynamic range of a double monochromator (−65 dB at 1550 nm) with the sensitivity of a single monochromator (−90 dBm at 1550 nm), as well as polarization insensitivity. However, moving parts, as in the direct-drive motor system for grating tuning, generally make the mechanism sensitive to vibrations and shock. In addition, an internal adsorption cell helps in wavelength calibration. The majority of units in laboratories today utilize this configuration.        
Fixed Grating/Scanned Detector OSA, Shown in FIG. 2b.                 In these OSAs the detector is scanned against a stationary grating. This reduces the number of moving parts, making it less prone to shock and motion; however, at the expense of a reduced wavelength range of 1525 to 1570 nm. The moving detector also slows the data acquisition and integration cycles. Typical resolution bandwidth of 0.1 to 0.5 nm, amplitude measurement accuracy of <0.8 dB and small size make it convenient for characterizing WDM networks.        
Scanning Michelson Wavelength Meter, Shown in FIG. 2c.                 By counting the number of fringes as one arm of a Michelson interferometer is extended, one can measure the wavelength to a very high degree of accuracy. In the case of multiple wavelengths, counting fringes is insufficient to extract their spectral profile. However, by measuring the amplitude of these fringes as the interferometer arm is extended, one can calculate the full spectrum of the input by performing a fast Fourier transform (FFT) calculation of these amplitudes.        This approach has the advantage of wide wavelength range (700 to 1650 nm) and wavelength accuracy of 10−2 to 10−4 nm for a single input wavelength, as well as 0.16 nm resolvable separation between input lines and power measuring accuracy of <1 dB for multiple input wavelengths. The response time of the instrument, however, is reduced due to the combination of a scanning mechanism, integration time and FFT analysis.        
Scanning Fabry-Perot Interferometer (FPI), Shown in FIG. 2d.                 An FPI is composed of two parallel and closely spaced (30 to 50 μm) mirrors, separated by a piezoelectric (PZT) spacer. By applying a voltage to the PZT, the FPI mirror separation changes, allowing light to be transmitted through it if mirror spacing is a multiple of half wavelength of the input. However, PZTs are inherently prone to drift causing these peaks to also drift. To account for drift, FPls require an additional independent reference for wavelength calibration such as an internal absorption cell. Alternatively, an external capacitor may be added as shown in FIG. 2d for mirror spacing measurement.        This capacitive micrometry approach, in combination with a high resolution FPI, can be used to produce a compact, solid state and board mountable device, having no moving parts. The spectral transmission characteristic of FPls has, however, a limited rejection for wavelengths adjacent to the peak, which limits the dynamic range and SNR measurements. To improve isolation better than 25 dB at 0.8 nm (with spectral range of 40 nm), the FPI requires Finesse values>350, where Finesse=spectral range/resolution, or a multi-pass configuration to improve rejection. The wavelength spectrum is developed by scanning the mirrors and averaging over the FPI spectral range.        
All the technologies discussed above process channels serially with an internal wavelength scan. None of the above described systems suggest how to implement a solid state design without increasing the response time of the device or limiting the wavelength range monitored by the device, nor how to process the channels of the signal in parallel to provide simultaneous processing of the channels. A network monitor and method of monitoring an optical network implementing a solid state design which allows parallel processing of the channels of the optical signal suitable as a network element and as a network service instrument for debugging and installation has not been taught, nor has such a device been successfully commercialized.